1. Field of the Invention
The present invention relates to an active material for a battery and a method of preparing the same, and more specifically to an active material for a battery with excellent electrochemical characteristics and thermal stability, and a method of preparing the same.
2. Description of the Related Art
Recently, in relation to trends toward more compact and lighter portable electronic equipment, there has been a growing need to develop a high performance and large capacity battery to be used for electric power for portable electronic equipment. Also, there has been extensive research on batteries with good safety characteristics and low cost.
Generally, batteries are classified as primary batteries that can be used only once and secondary batteries that are rechargeable. Primary batteries include manganese batteries, alkaline batteries, mercury batteries, silver oxide batteries and so on, and secondary batteries include lead-acid storage batteries, Ni-MH (nickel metal hydride) batteries, nickel-cadmium batteries, lithium metal batteries, lithium ion batteries, lithium polymer batteries and lithium-sulfur batteries.
These batteries generate electric power by using materials capable of electrochemical reactions at positive and negative electrodes. Factors that affect battery performance characteristics such as capacity, cycle life, power capability, safety and reliability, include electrochemical properties and thermal stability of active materials that participate in electrochemical reactions at the positive and negative electrodes. Therefore, research to improve the electrochemical properties and thermal stability of the active materials at the positive and negative electrodes continues.
Among the active materials currently being used for negative electrodes of batteries, lithium metal provides both high capacity because it has a high electric capacity per unit mass and high voltage due to a relatively high electronegativity. However, since it is difficult to assure the safety of a battery using lithium metal, other materials that can reversibly deintercalate and intercalate lithium ions are being used extensively for the active material of the negative electrodes in lithium secondary batteries.
Lithium secondary batteries use materials that reversibly intercalate or deintercalate lithium ions during charge and discharge reactions for both positive and negative active materials, and contain organic electrolyte or polymer electrolyte between the positive electrode and the negative electrode. This battery generates electric energy from changes of chemical potential during the intercalation/deintercalation of lithium ions at the positive and negative electrodes.
Lithium metal compounds of a complex formula are used as the positive active material of the lithium secondary battery. Typical examples include LiCoO2, LiMn2O4, LiNiO2, LiNi1-xCoxCo2 (0<x<1), LiMnO2 and a mixture of these compounds. Manganese-based positive active materials such as LiMn2O4 or LiMnO2 are the easiest to synthesize, less costly than the other materials, and environmentally friendly. However, these manganese-based materials have relatively low capacity. LiCoO2 has good electric conductivity, high battery voltage and excellent electrode characteristics. This compound is presently the most popular material for positive electrodes of commercially available Li-ion batteries. However, it is relatively expensive and has low stability during charge-discharge at a high rate. LiNiO2 is currently the least costly of the positive active materials mentioned above and has a high discharge capacity, but it is difficult to synthesize and the least stable among the compounds mentioned above.
The above active materials are lithiated intercalation compounds in which stability and capacity of active material is determined by the nature of intercalation/deintercalation reactions of lithium ions. As the charging potential increases, the amount of Li deintercalation increases, thus increasing the electrode capacity, but thermal stability of the electrode decreases steeply due to its structural instability. When the interior temperature of the battery increases in the fully charged state, the bonding energy between the metal ions and the oxygen of the active material decreases, releasing oxygen when a temperature above a threshold value is reached. For example, LiCoO2 active material in a charged state has the formula Li1-xCoO2, where 0<x<1. Because the active material having the above structural formula is unstable, especially when x>0.5, if the interior temperature of the battery increases beyond the threshold value, oxygen gas (O2) is released. Since the reaction of this oxygen with organic electrolyte in the battery is highly exothermic, a thermal runaway situation may be created in the battery, and this may cause an explosion in the battery. Therefore, it is desirable to control the threshold temperature and the amount of exothermic heat evolved from the reaction in order to improve the safety of the battery.
One way of controlling the threshold temperature and the amount of exothermic heat is controlling the surface area of the active material through particle size control, which is usually achieved by pulverizing and sieving the active material. The smaller the particle size, i.e. the larger the surface area, the better the battery performance, in particular the power capability, i.e. capacity values and discharge voltages at low temperatures and at high rates. However, battery safety, cycle life and self-discharge become worse as the particle size decreases. Because of these conflicting factors, there is a practical limitation in controlling the threshold temperature and heat evolution rate through particle size alone.
In order to improve stability of active material itself during charge-discharge, it has been suggested to dope other elements into the Ni-based or Co-based lithium oxide. For example, U.S. Pat. No. 5,292,601 discloses LixMO2 (where M is at least one element selected from Co, Ni and Mn; and x is 0.5 to 1) as an improved material over LiCoO2.
Another attempt to improve stability includes modifying the surface of the active material. Japanese Patent Laid-Open No. Hei 9-55210 discloses that lithium nickel-based oxide is coated with alkoxide of Co, Al and Mn and is heat-treated to prepare a positive active material. Japanese Patent Laid-Open No. Hei 11-16566 discloses lithium-based oxide coated with a metal and/or an oxide thereof. The metal includes Ti, Sn, Bi, Cu, Si, Ga, W, Zr, B or Mo. Japanese Patent Laid-Open No. Hei 11-185758 discloses coating a surface of lithium manganese oxide with a metal oxide by using a co-precipitation process and heat-treating the same to prepare a positive active material.
However, the above methods did not solve the fundamental problems associated with the safety of the battery: The threshold temperature wherein the active material prepared according to the above methods begins to react with an electrolyte, that is, the decomposition temperature, at which oxygen bound to metal of the active material begins to be released (exothermic starting temperature, Ts) does not shift sufficiently to a higher temperature and the amount of released oxygen (the value related to the exothermic heat) does not decrease sufficiently by the methods described above.
The structural stability of positive active material having the composition of Li1-xMO2 (M═Ni or Co) during charging is strongly influenced by the value of x. That is, when 0<x<0.5, cyclic stability is steadily and stably maintained, but when x is greater than or equal to 0.5, phase transition occurs from a hexagonal phase to a monoclinic phase. This phase transition causes an anisotropic volume change, which induces development of micro-cracks in the positive active material. These micro-cracks damage the structure of the active material, and thus the battery capacity decreases dramatically and the cycle life is reduced. Therefore, when anisotropic volume change is minimized, the capacity and the cycle life of the battery are improved.
In order to increase structural stability of positive active material, U.S. Pat. No. 5,705,291 discloses a method in which a composition comprising borate, aluminate, silicate or mixtures thereof was coated onto the surface of a lithiated intercalation compound, but it still has a problem with structural stability.
In the above description, positive active materials of lithium secondary batteries and related examples of developments were explained. Recently, in relation to the tendency to develop portable electronic equipment that is more compact and lightweight, other types of batteries have the same demands for an active material that guarantees battery performance, safety and reliability. Research and development is therefore accelerated on electrochemical properties and thermal stability of positive active materials to ensure improved performance, safety and reliability of batteries.